BUG #2178: NOT IN command don't work

The following bug has been logged online:

Bug reference:      2178
Logged by:          Daniel Afonso Heisler
Email address:      daniel@solis.coop.br
PostgreSQL version: 8.1.X
Operating system:   Linux
Description:        NOT IN command don't work
Details: 

When i run the following query, postgreSQL return TRUE.
 # SELECT true WHERE 1 NOT IN (2,3);

But, when i run the next query, it don't return TRUE
 # SELECT true WHERE 1 NOT IN (2,NULL,3);

In theory, that is not correctly. The operator IN work with normally. See
the next example:

 # SELECT true WHERE 1 IN (1,2,NULL,3);